I think the "best" luff tension setup for any given condition depends on too many different variables for a simple answer. Your sailmaker should be able to tell you what distance from pole to mast he designed the spi for, that would be the best starting point. Then experiment from there with two boat tuning.
On the Tornado we set up the distanse from pole to mast to be 9100mm whereas the max luff length is 9150mm (which the sailmakers never cut that long, as they know the cloth will stretch some and they need some margins of measurement/assembly error). 20-30mm of slack in the luff was our aim, or grab the luff and twist about 35-45deg before the luff becomes tight. This was on the Gran Segel model-4 spi. I am certain this will differ between sailmakers according to how they designed the luff and the rest of the sail.
If I had the money, I would definately have ordered a F-16 spi from Gran Segel in Malmø! As it is, I think most sailmakers cut their beachcat spis too deep with too much shape in the luff. A bit less shape and draft means the sail will become faster for less drag, but it might be bad for VMG if overdone. Only testing will tell, but the Gran Segel spis with their flat shapes and twist was a leap ahead on the Tornado when launched at the worlds at Marthas Vineyard in 2002 (not that I was there, but friends was..).